Summarize data

Please, check my code!
I forgot to include some kind of readme.st. To test custom command:
python manage.py summarize [date YYYY-MM-DD] [--nodeletion]
If you pass no parameters it summarizes current date minus 15 days (single day and no a 15-days period)
Nitpicking: Since the function returns a boolean, I usually like to name it in a way that it's obvious it returns such.  E.g. ```is_valid_date```.